Cokriging

[n0228a]

had to move cokriging to new branch and start pull request again

[LSchueler]

For future references, this is a follow up of PR #394

[n0228a]

I let Claude ultrareview look for errors and corrected some architectural things, no algorithmic logic was touched: src/gstools/cokriging/base.py

final result — fixing self.field storing SK instead of cokriging , normalizer mixing , only_mean crash/corruption , and unnecessary variance solve
for ICCK

• _apply_simple_collocated: removed the spurious + kλm_Z mean term (residual-space arithmetic needs no explicit mean)
• _apply_intrinsic_collocated: computes lambda_Y0 = C_YZ0/C_Y0 locally from _compute_covariances() instead of reading fragile instance state
• _summate: full rewrite — computes sk_weights = krige_mat @ k_vec once in NumPy, derives field and variance directly, removes duplicate Cython call; drops dead
  ndim==1 branch, write-only _secondary_at_primary, and unreachable else: raise
• _prepare_secondary: validates secondary_data size against eval grid, raises ValueError with clear message on mismatch
• init (intrinsic block): validates secondary_cond_pos matches cond_pos; applies isfinite mask to secondary_cond_val before super().init so NaN-filtered
  primaries don't cause broadcast crash
• secondary_data is None guard: message now names CondSRF

src/gstools/cokriging/methods.py

• Removed duplicate isinstance(correlogram, Correlogram) checks from SimpleCollocated and IntrinsicCollocated (base already checks); removed now-unused import

src/gstools/init.py

• Added Correlogram and CollocatedCokriging to top-level imports and all

tests/test_cokriging.py — 11 new tests:

• test_scck_field_matches_full_system / test_icck_field_matches_full_system — independent full-system oracles locking the math
• test_field_stored_is_cokriging_result — self.field holds cokriging, not SK
• test_only_mean_not_supported — raises NotImplementedError
• test_zero_corr_with_normalizer_equals_sk — normalizer round-trip correctness
• test_secondary_data_wrong_length / test_secondary_data_structured_mesh — shape validation
• test_icck_nan_in_cond_val / test_icck_non_collocated_secondary_rejected — conditioning validation
• test_condsrf_gives_clear_error — CondSRF error message
• test_icck_summate_matches_oracle_with_chunks / test_top_level_exports — efficiency rewrite and export check